The Vehicle Speed Sensor (VSS) is located on the transfer case for 4WD models or on the transmission for 2WD models. It produces a pulsing voltage signal that is translated by the PCM into miles-per-hour, which is used to operate the cruise control system, the speedometer, and the torque converter clutch and shift solenoid in the transmission. A defective VSS can cause various driveability and transmission problems. On O8D-I vehicles, the VSS is a permanent magnet generator triggered by a toothed rotor on the transmission output shaft. On OBD-11vehicles, the VSS is a hall-effect switch. To check the VSS, raise the vehicle and support it securely on jackstands. Locate the VSS and unplug the electrical connector from it. Inspect the connector terminals and the VSS wire harness for any damage. On OBD-I vehicles, use a digital voltmeter to back-probe the two wire terminals of the VSS connector. The sensor should produce a minimum of 0.5 volts and the voltage should increase as the transmission output shaft rotates faster. If the VSS is defective, replace it. Testing the VSS on OBD-II vehicles is beyond the scope of the home mechanic. To replace the VSS, raise the vehicle and support it securely on jackstands. Disconnect the electrical connector from the VSS. Remove the hold-down bolt and the hold-down clamp and pull the VSS out of the transfer case or transmission. Whether you're installing the old VSS or a new unit, be sure to replace the O-rings. Tighten the hold down bolt to the torque. Installation is otherwise the reverse of removal.